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1. Graphene Film Growth on Silicon Carbide by Hot Filament Chemical Vapor Deposition

   https://doi.org/10.3390/nano12173033  

   Rodríguez-Villanueva, Sandra; Mendoza, Frank; Weiner, Brad R.; Morell, Gerardo (September 2022, Nanomaterials)

   The electrical properties of graphene on dielectric substrates, such as silicon carbide (SiC), have received much attention due to their interesting applications. This work presents a method to grow graphene on a 6H-SiC substrate at a pressure of 35 Torr by using the hot filament chemical vapor deposition (HFCVD) technique. The graphene deposition was conducted in an atmosphere of methane and hydrogen at a temperature of 950 °C. The graphene films were analyzed using Raman spectroscopy, scanning electron microscopy, atomic force microscopy, energy dispersive X-ray, and X-ray photoelectron spectroscopy. Raman mapping and AFM measurements indicated that few-layer and multilayer graphene were deposited from the external carbon source depending on the growth parameter conditions. The compositional analysis confirmed the presence of graphene deposition on SiC substrates and the absence of any metal involved in the growth process. 

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2. Raman enhancement induced by exciton hybridization in molecules and 2D materials

   https://doi.org/10.1038/s41699-024-00446-z  

   Kitadai, Hikari; Tan, Qishuo; Ping, Lu; Ling, Xi (December 2024, npj 2D Materials and Applications)

   Abstract Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for trace-level fingerprinting. Recently, layered two-dimensional (2D) materials have gained significant interest as SERS substrates for providing stable, uniform, and reproducible Raman enhancement with the potential for trace-level detection. Yet, the development of effective 2D SERS substrates is still hindered by the lack of fundamental understanding of the coupling mechanism between target molecules and substrates. Here, we report a systematic excitation-dependent Raman spectroscopy investigation on the coupling between 2D materials such as SnS₂, MoS₂, WSe₂, and graphene and small organic molecules like rhodamine 6G (Rh 6G). Strong coupling between SnS₂and Rh 6G is found due to their degenerate excitons through Raman excitation profiles (REP), leading to the enhancement of Rh 6G vibrational modes that are observable down to 10⁻¹³M. Our study shows that exciton coupling in the substrate-adsorbate complex plays a vital role in the Raman enhancement effect, opening a new route for designing SERS substrates for high sensitivity. 

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3. MoS2 Nanodonuts for High-Sensitivity Surface-Enhanced Raman Spectroscopy

   https://doi.org/10.3390/bios11120477  

   Ghopry, Samar Ali; Sadeghi, Seyed M.; Berrie, Cindy L.; Wu, Judy Z. (December 2021, Biosensors)

   Nanohybrids of graphene and two-dimensional (2D) layered transition metal dichalcogenides (TMD) nanostructures can provide a promising substrate for extraordinary surface-enhanced Raman spectroscopy (SERS) due to the combined electromagnetic enhancement on TMD nanostructures via localized surface plasmonic resonance (LSPR) and chemical enhancement on graphene. In these nanohybrid SERS substrates, the LSPR on TMD nanostructures is affected by the TMD morphology. Herein, we report the first successful growth of MoS2 nanodonuts (N-donuts) on graphene using a vapor transport process on graphene. Using Rhodamine 6G (R6G) as a probe, SERS spectra were compared on MoS2 N-donuts/graphene nanohybrids substrates. A remarkably high R6G SERS sensitivity up to 2 × 10−12 M has been obtained, which can be attributed to the more robust LSPR effect than in other TMD nanostructures such as nanodiscs as suggested by the finite-difference time-domain simulation. This result demonstrates that non-metallic TMD/graphene nanohybrids substrates can have SERS sensitivity up to one order of magnitude higher than that reported on the plasmonic metal nanostructures/2D materials SERS substrates, providing a promising scheme for high-sensitivity, low-cost applications for biosensing. 

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4. Selective area multilayer graphene synthesis using resistive nanoheater probe

   https://doi.org/10.1038/s41598-023-34202-y  

   Torres, Ingrid; Aghaei, Sadegh Mehdi; Pala, Nezih; Gaitas, Angelo (December 2023, Scientific Reports)

   Abstract Graphene has been a material of interest due to its versatile properties and wide variety of applications. However, production has been one of the most challenging aspects of graphene and multilayer graphene (MLG). Most synthesis techniques require elevated temperatures and additional steps to transfer graphene or MLG to a substrate, which compromises the integrity of the film. In this paper, metal-induced crystallization is explored to locally synthesize MLG directly on metal films, creating an MLG-metal composite and directly on insulating substrates with a moving resistive nanoheater probe at much lower temperature conditions (~ 250 °C). Raman spectroscopy shows that the resultant carbon structure has properties of MLG. The presented tip-based approach offers a much simpler MLG fabrication solution by eliminating the photolithographic and transfer steps of MLG. 

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5. Ferroelectric surface photovoltage enhancement in chromium-doped SrTiO ₃ nanocrystal photocatalysts for hydrogen evolution

   https://doi.org/10.1039/D0MA00463D  

   Assavachin, Samutr; Nail, Benjamin A.; V. Goncalves, Renato; Mulcahy, Justin R.; Lloyd, Sarah E.; Osterloh, Frank E. (August 2020, Materials Advances)

   Chromium-doped SrTiO 3 nanocrystals of perovskite structure type and 45 nm (±15 nm) edge lengths were obtained by hydrothermal synthesis in water from titanium oxide, strontium hydroxide, and chromium( iii ) nitrate. According to XPS, the majority of the surface chromium (68.3%) is present in the 3+ state and the remainder (32.2%) in the 6+ state. Optical spectroscopy confirms a broad absorption at 2.3–2.9 eV from Cr(3+) dopant states, in addition to the 3.2 eV band edge of the SrTiO 3 host. After modification with Pt nanoparticles, Cr-doped SrTiO 3 nanocrystals catalyze photochemical H 2 evolution from aqueous methanol under visible light illumination (>400 nm) and with an apparent quantum yield of 0.66% at 435 nm. According to surface photovoltage spectroscopy (SPS), Cr-doped SrTiO 3 nanocrystals deposited onto gold substrates are n-type and have an effective band gap of 1.75 eV. SPS and transient illumination experiments at 2.50 eV reveal an anomalous surface photovoltage that increases with prior light exposure to values of up to −6.3 V. This photovoltage is assigned to ferroelectric polarization of the material in the space charge layer at the Au/SrTiO 3 :Cr interface. The polarization is stable for 24 h in vacuum but disappears after 12 h when samples are stored in air. The electric polarizability of SrTiO 3 :Cr is confirmed when films are exposed to static electric fields (1.20 MV m −1 ) in a fixed capacitor configuration. The discovery of a ferroelectric effect in Cr-doped SrTiO 3 could be significant for the development of improved photocatalysts for the conversion of solar energy into fuel. 

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